The invention relates to a display screen, in particular a color display screen for a cathode ray tube, comprising a phosphor layer which includes a phosphor composition of a phosphor with a coating comprising a water-soluble organic binder. The invention also relates to a phosphor composition of a phosphor with a coating comprising a water-soluble organic binder.
Display screens of cathode ray tubes are provided on the inner surface with structured phosphor layers exhibiting dot-shaped or stripe-shaped patterns. In the case of color display screens, each phosphor element of the phosphor layer is composed of a triad of phosphor elements, each phosphor element emitting one of the three primary colors red, blue and green. This structured phosphor layer is customarily applied to the screen glass by means of a photochemical process in which phosphor suspensions are used in the wet-chemical variant, and phosphor lacquers are used in the dry variant. In the wet-chemical process for coating screen glass with phosphors, aqueous phosphor suspensions are used which include, for example, polyvinyl alcohol (PVA) as the binder and ammonium dichromate (ADC) as the light-sensitive component.
In the wet-chemical display screen process, the properties of the phosphor particles are substantially influenced by the surface properties of the phosphor powders from which the phosphor suspensions are prepared, irrespective of whether the phosphor particles are processed in suspension or the phosphor layer is formed on the display screen glass. To control the surface properties of the phosphor powders, a method which yields satisfactory results, in view of the display screen process, includes providing the inorganic phosphor powders, after their manufacture, with chemically different coatings. Besides, pigmentation of phosphors for improving the contrast in the display screen generally requires the use of an additional binding agent which ensures that the adhesion of the pigment on the phosphor material is satisfactory.
EP 0 549 327 discloses a phosphor for a cathode ray tube, which phosphor comprises phosphor particles and a surface-treating substance which is provided on the phosphor particles, which substance is composed of a mixture including a metal alginate, which contains at least a type of a metal selected from the group formed by zinc, aluminum and an alkaline earth metal, and at least a type of a water-soluble binder selected from the group formed by Arabic gum, gelatin, polymethacrylamide and polyvinyl alcohol.
This phosphor has the drawback that the sieving properties, the adhesion of the pigment to the phosphor as well as the adhesion of the structured phosphor layer to the screen glass are sub-optimal.
At present, however, ever higher requirements are imposed on the processability of phosphor powders in suspension as well as on the quality of display screens regarding brightness, color purity, absence of fog, definition and adhesion of the structured phosphor coating.
Therefore, it is an object of the invention to provide a display screen comprising a phosphor layer, which display screen exhibits optimum values as regards brightness, color purity, absence of fog, definition and adhesion of the phosphor material to the screen glass.
In accordance with the invention, this object is achieved by a display screen comprising a phosphor layer including a phosphor composition of a phosphor with a coating comprising a water-soluble organic binder, which is selected from the group formed by the polyurethanes, polyacrylamides, polyamide resins, etherified melamine compounds and melamine-formaldehyde compounds and etherified urea-formaldehyde resins, and comprising oxygen compounds of one or more elements selected from the group formed by the alkaline earth elements and zinc.
A display screen in accordance with the invention is characterized by a surprising improvement in display-screen brightness, color purity and adhesion of the phosphor to the screen glass. The phosphor layer further exhibits a high packing density of the phosphor material in the phosphor layer and hence optimum brightness of the screen. The phosphor layer further exhibits a good homogeneity, i.e. prevention of structures owing to the flow properties of the suspension, and is free of xe2x80x9cpinholesxe2x80x9d. In addition, the phosphor layer is sharply defined and the thickness of the phosphor layer is homogeneous. The shadow mask structure is imaged with high precision onto the structured phosphor layer. The individual phosphor elements are sharply defined and adhere well to the screen glass. Fog formation in the non-exposed screen area of the phosphor layer is low.
Within the scope of the invention it is preferred that the phosphor composition includes polycarboxylates, gelatin or silicon dioxide.
It is also preferred that the solids content of the water-soluble organic binder in the phosphor layer ranges from 0.01% by weight to 1.0% by weight with respect to the phosphor, that the content of the oxygen compounds of the alkaline earth elements and zinc in the phosphor layer ranges from 0.01% by weight to 0.80% by weight with respect to the phosphor, and that the silicon dioxide content, calculated as SiO2, in the phosphor layer ranges from 0.01% by weight to 1.0% by weight with respect to the phosphor.
Within the scope of the invention it is particularly preferred that the water-soluble organic binder has a molecular mass below 300,000, particularly below 100,000.
Within the scope of the invention it is particularly preferred that the organic binder is a polyurethane having a molecular mass below 20,000. Such a phosphor composition is characterized by a very good sieving behavior.
It may furthermore be preferred that the oxygen compound is a zinc-oxygen compound.
Within the scope of the invention it may be preferred that the silicon dioxide is composed of silicates or of colloidal SiO2 particles having an average particle size of 4 nm less than d less than 150 nm in accordance with a specific surface area of 25 m2/g less than As less than 550 m2/g.
It may also be preferred that the weight ratio of the SiO2 particles (calculated as SiO2) relative to the overall quantity of the metal-oxygen compounds of the alkaline earth elements and zinc lies in the range from 1:10 to 30:1, and the weight ratio of the SiO2 particles relative to the overall quantity of the organic binder lies in the range from 1:30 to 30:1.
It may also be preferred that the coating includes 0.01% by weight to 15% by weight of a color filter pigment.
Within the scope of the invention it is further particularly preferred that the water-soluble organic binder comprises at least a compound selected from the group formed by the polyacrylamides and gelatins, and at least a compound selected from the group formed by the etherified melamine compounds and melamine-formaldehyde compounds and the etherified urea-formaldehyde resins, the weight ratio of the compounds selected from the group formed by the polyacrylamides and gelatins relative to the overall quantity of the compounds selected from the group formed by the etherified melamine compounds and melamine-formaldehyde compounds and the etherified urea-formaldehyde resins ranging from 1:5 to 20:1.
Such a phosphor layer is characterized by a sharp definition of the structured phosphor layer and a very good adhesion. In addition, pigment particles bonded to the phosphor grains in the manner described hereinabove exhibit a very good adhesion to the phosphor.
The invention also relates to a phosphor composition of a phosphor with a coating comprising a water-soluble organic binder, which is selected from the group formed by the polyurethanes, polyacrylamides, polyamide resins, etherified melamine compounds and melamine-formaldehyde compounds, etherified urea-formaldehyde resins, and comprising oxygen compounds of one or more elements selected from the group formed by the alkaline earth elements and zinc. Such a phosphor composition can be readily processed as it exhibits a very good sieving behavior of the powder and a high filterability of the suspension also after a prolonged period of time and at lower pH-values. The extremely good dimensional stability of the suspension of phosphors coated in accordance with the invention is demonstrated by the good stability of the particle-size distribution upon adding electrolyte solution and treating with ultrasound. Further advantages reside in the improved adhesion of the phosphor to the display screen glass, the very small degree of fog-formation, the high color purity as a result of the excellent adhesion of the pigment to the phosphor grains, as well as the high packing density and very good homogeneity of the relevant phosphor layers.
In the suspension, the phosphor composition exhibits a high stability, which manifests itself as a high filterability, a negligibly small quantity of loose pigment material as well as insensitivity of the particle-size distribution to the addition of electrolyte solution and to an ultrasound treatment for a prolonged period of time at critical pH-values. Said phosphor composition is also characterized by a very good sieving behavior.